Hand-Guided or Stationary Power Tool Having a Drive Unit

ABSTRACT

A hand-guided or stationary power tool has a drive unit having a motor that includes a rotor having a permanent magnet and a stator and has a motor control designed to trigger the motor in a first rotational speed range according to a voltage-controlled mode and to trigger the motor in a second rotational speed range following the first rotational speed range in the direction of a higher rotational speed according to a field-weakening operation.

FIELD OF THE INVENTION

The present invention relates to a hand-guided or stationary power toolhaving a drive unit, in particular a battery-operated power tool, e.g.,a cordless screwdriver.

BACKGROUND INFORMATION

There are various known power tools that may be operated using anelectric motor. In some cases these power tools are provided with ECmotors that are brushless, i.e., the rotor is provided with permanentmagnets that rotate in a magnetic field generated by stator coils. Therotational speed of the EC motor is usually controlled via the appliedmotor voltage.

The motor voltage is usually applied by pulse width modulation of thepower transistors of the drive unit. In doing so, the motor iscommutated as a function of fixed rotor positions. The EC motor isusually designed for the maximum rotational speed of the power tool.Because this is usually associated with a lower number of requiredwindings, it results in a comparatively low torque in relation to thestator current. This current is determined by the limiting values of thepower electronics, the motor and/or the battery. The torque is thus alsolimited to this maximum value over the entire rotational speed range.

This behavior is unfavorable for use of such EC motors in power tools.In this situation, it is often necessary for a high torque to beavailable at a low rotational speed and for a low torque to be availableat a high rotational speed. With today's power tools, this is achievedby providing reversible gears with which a high torque at a highrotational speed is converted into a lower rotational speed of the toolto achieve a high torque at a lower rotational speed. However, it iscomplex and expensive to provide a reversible gear, which also resultsin friction losses that increase the power tools' power consumption.

In the past, power tools have been designed for a high rotational speed,so the stator coils have a low ohmic resistance and a lower inductance,which may result in a relatively high current when the coils areshort-circuited, and consequently the motor or the electronics may bedamaged. In the event of a fault or an inadmissible operating state,which may occur, for example, when a battery is removed from abattery-operated power tool when the motor is rotating, the motor and/orthe electronics may be destroyed due to short circuiting of the motor.For this reason, measures are usually provided to prevent damage to themotor.

An object of the present invention is to provide an improved power toolthat meets the requirements of a high torque at a lower rotational speedand a lower torque at a high rotational speed without using a reversiblegear and whereby the power tool is more resistant to damage due tofaults or inadmissible operating states.

SUMMARY OF THE INVENTION

According to the present invention, a power tool having a drive unit isprovided, including a motor having a rotor with a permanent magnet and astator. The drive unit also has a motor control designed to trigger themotor in a first rotational speed range according to avoltage-controlled mode and to trigger the motor in a second rotationalspeed range following the first rotational speed range in the directionof a higher rotational speed according to a field-weakening operation.

With the power tool according to the present invention, the motorrotational speed is to be adjusted via the voltage-controlled mode aswell as via the field-weakening operation. This has the advantage thatthe motor no longer needs to be designed electrically for a maximumrotational speed but instead may be designed for a medium rotationalspeed, so that a correspondingly higher torque may be achieved at thesame maximum voltage in the range up to the medium rotational speed thanwith a traditional motor designed for a higher rotational speed. Highertorques are thus possible in the lower (first) rotational speed range.The motor is operated in field-weakening operation to achieve a highrotational speed beyond the medium rotational speed. In field-weakeningoperation, there is a change in the phase relation of the rotor fieldand the stator field, so that the rotational speed may be increased to arotational speed that is higher than the maximum rotational speed involtage-controlled operation. The higher rotational speed is achieved ata reduced torque in comparison with the torque at the maximum voltage.This operating performance is advantageous for power tools because theyrequire either a high torque at a low rotational speed or a highrotational speed at a low torque. The drive unit may thus cover workingranges which are possible with conventional drives or conventionallyoperated EC motors only by using corresponding reversible gears. Due tothe motor being designed for a comparatively low motor rotational speed,i.e., a motor rotational speed lower than the maximum rotational speedat which the power tool is to be operated at maximum speed, theresistance and inductance of the stator winding are higher in comparisonwith those of a traditional power tool. The stator winding may thus bedesigned to be short-circuit-proof. The motor may thus beshort-circuited by the electronics for immediate stoppage in anyoperating state without the risk of damaging the motor or electronicsdue to short-circuit currents. In a fault scenario or in inadmissibleoperating states caused by sudden removal of the power supply when themotor is rotating, for example, short-circuiting of the motor by thepower electronics of the drive is thus sufficient to leave the dangerousoperating states by the fastest way possible.

In field-weakening operation, the time characteristic of the motorcurrents approaches the sinusoidal form increasingly and thus reducesthe increased pulsation losses in the magnet and iron of the motor, sothe power tool may be operated more efficiently.

Another advantage is that the stator windings are provided with a largenumber of windings so that a smaller wire diameter may be selected forthe windings, thereby simplifying production of the motor.

According to one specific embodiment of the present invention, the motorcontrol applies a voltage to the motor in voltage-controlled mode topreselect the rotational speed of the motor. The first rotational speedrange is determined by the range between 0 and the medium preselectedrotational speed, which is achieved by applying a maximum voltage involtage-controlled mode.

The motor control of the power tool may have a phase shifter to adjustfield-weakening operation through the phase shift between the statormagnetomotive force and the rotor magnetomotive force. The phase shiftermay be designed in particular to adjust the phase between the statormagnetomotive force and the rotor magnetomotive force (between electricloading of the stator and electric loading of the rotor) to be greaterthan 90° in field-weakening operation.

According to a preferred specific embodiment, the power tool has a powersupply in the form of a rechargeable battery. In particular, the powertool may be designed as a cordless screwdriver, whereby a shaft of themotor is coupled to a machining tool by a gear having a fixed gearratio, i.e., the power tool is designed without a reversible gear.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a drive unit for a power tool.

FIG. 2 shows vector diagrams to illustrate voltage-controlled operationand field-weakening operation.

FIG. 3 shows a diagram to illustrate the plot of torque as a function ofrotational speed with a traditional power tool and with a power toolaccording to one specific embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a drive unit 1 for a power tool, in particular for abattery-operated power tool, in particular a cordless screwdriver. Thedrive unit has a motor 2 which is supplied with power via powerelectronics 3. Motor 2 is triggered by pulse-width-modulated voltagepulses generated with the help of transistor bridges, producing aneffective voltage on motor 2. Motor 2 is designed as a three-phase motorhaving three stator windings 4, each triggered by one of the transistorbridges.

Power electronics 3 is triggered via a motor control 5, generating PWMtriggering signals for power electronics 3 in a PWM unit 6. PWMtriggering signals are set in PWM unit 6 as a function of the positionof a rotor (not shown) of motor 2 and as a function of a setpoint valueexternally preselected with the help of a regulating unit 7.

The setpoint value is directly or indirectly preselected by a user withthe help of an operating element.

The position of the rotor of motor 2 is measured by a position sensor 8so that information about the absolute position of the rotor isavailable in regulating unit 7. Position sensor 8 measures the positioneither absolutely or in relation to a previous position, the absoluteposition being determined in regulating unit 7 in this case via thechanges in position.

Regulating unit 7 is designed in such a way that two modes areimplemented. In a first mode, motor 2 is triggered according to avoltage-controlled mode, so that the rotational speed, in particular therotational speed at no load or at a constant load, is proportional tothe effective voltage applied via power electronics 3. In a second mode,which is determined by a rotational speed range above a preselectedrotational speed, regulating unit 7 switches to the so-calledfield-weakening operation during regulating the power electronics fortriggering motor 2. A phase shifter 9 is provided for this purpose,ensuring that the phase relation between the rotor field supplied by therotor and the exciting field created by the stator is greater than 90°,the degree of phase shifting determining the rotational speed.

Motor control 5 may be designed in the form of a microcontrollersupplying the triggering signals of power electronics 3. Drive unit 1may be supplied with power via a battery 10, in particular arechargeable battery. Regulating unit 7 may continue to detect thestator currents and the voltages in power electronics 3 to obtaininformation about the load applied to motor 2.

FIG. 2 shows vector diagrams for the stator and rotor magnetomotiveforce for voltage-controlled operation as well as for field-weakeningoperation. The vector diagram on the left represents the status involtage-controlled operation. In this mode, the maximum achievable motorrotational speed is fixedly preselected by the level of the supplyvoltage applied to the power electronics. The vector of the statorcurrent and/or stator magnetomotive force θ₁ is thus almostperpendicular to the vector of the magnetic equivalent magnetomotiveforce of rotor θ_(PM). The rotational speed may be adjusted almostproportionally by varying the applied voltage on the stator windings.

The vector diagram on the right side of FIG. 2 represents the status ina field-weakening operation. The vector of stator magnetomotive force θ₁and the vector of magnetic equivalent magnetomotive force θ_(PM) of therotor are no longer mutually perpendicular but instead the stator androtor magnetomotive force each now have an angle of much more than 90°.When the stator magnetomotive force is broken down vectorially into acomponent parallel to the rotor magnetomotive force (X axis) and acomponent perpendicular to that (Y axis), it is thus apparent that theX-axis component of stator magnetomotive force θ_(1,x) is opposite therotor magnetomotive force and thus diminishes its effect. This effect isa reduction in synchronized internal voltage. The operative mechanismcorresponds to the mechanism known from a separately excited d.c.shunt-wound machine. If the excitation strength is reduced duringoperation of the machine, this causes a reduction in the rotationallyinduced armature voltage, resulting in an increase in armature currentand thus an accelerating torque. The machine accelerates until theinduced voltage and the voltage drop on the armature resistance are inequilibrium with the feed voltage. This allows the rotational speed ofthe d.c. machine to increase.

FIG. 3 shows the curves of torque as a function of motor rotationalspeed. With a traditional drive unit (dashed line) for a power tool of aconventional design having an EC motor, the available torque is largelyconstant up to the maximum torque, because it is determined by thecurrent limit of the electronics and the motor.

With the power tool according to the present invention (solid line), thevoltage-controlled mode is likewise selected in a first rotational speedrange, with the motor of the power tool being designed to obtain themaximum torque at a medium rotational speed nm, determined by themaximum available power and/or the maximum permissible current limit. Asa result, a higher torque is available at the same current level thanwith a traditional power tool even at a lower rotational speed in therange of 0 to n_(M) because of the greater number of windings. In thesecond rotational speed range, i.e., at a rotational speed between n_(M)and n_(Max) (maximum rotational speed), the motor is operated infield-weakening operation in which a lower torque is available than themaximum torque achieved at a medium rotational speed. The greater therotational speed of the motor, the lower the torque, decreasinginversely proportionally until reaching maximum rotational speedn_(Max).

The torque characteristic curve corresponds to a characteristic curvewhich is very suitable for operation of power tools. In traditionalpower tools, such a characteristic curve is usually achieved by using agear having reversible gear ratios, which provides high torques in thelower rotational speed range and lower torques in the high rotationalspeed range. The power tool according to the present invention thereforehas the advantage that it may be designed without a reversible gear andtherefore the friction losses due to the gear may be avoided.Furthermore, such a power tool allows a sturdier design of the statorwindings, which may be short-circuited in any operating state by theelectronics due to their greater resistance and inductance without therisk of damage to the motor or electronics. The higher resistance andinductance result from the fact that, at a given maximum voltage involtage-controlled operation, the motor may be designed for a lowerrotational speed because the higher rotational speed is achievable viafield-weakening operation.

1-9. (canceled)
 10. A power tool, the power tool being one ofhand-guided and stationary, the power tool comprising: a drive unitincluding: a motor including a rotor having a permanent magnet, and astator; and a motor control for triggering the motor in a firstrotational speed range according to a voltage-controlled mode and fortriggering the motor in a second rotational speed range following thefirst rotational speed range in a direction of a higher rotational speedaccording to a field-weakening operation.
 11. The power tool accordingto claim 10, further comprising an operating element for preselecting arotational speed at no load.
 12. The power tool according to claim 10,wherein the motor control applies a voltage to the motor in thevoltage-controlled mode to preselect a rotational speed of the motor,the first rotational speed range being determined by a range between 0and a medium rotational speed, which is achieved by applying a maximumvoltage in the voltage-controlled mode.
 13. The power tool according toclaim 10, wherein the motor control has a phase shifter to adjust aphase between a stator magnetomotive force and a rotor magnetomotiveforce in field-weakening operation.
 14. The power tool according toclaim 13, wherein the phase shifter adjusts the phase between the statormagnetomotive force and the rotor magnetomotive force to be greater than90° in field-weakening operation.
 15. The power tool according to claim10, further comprising a power supply in the form of a rechargeablebattery.
 16. The power tool according to claim 15, wherein the powertool is a cordless screwdriver.
 17. The power tool according to claim10, wherein the motor further includes a shaft coupled directly to amachining tool.
 18. A method for operating a drive unit for one of ahand-guided and stationary power tool, the drive unit including a motorwhich includes a rotor having a permanent magnet, and a stator, themethod comprising: triggering the motor according to avoltage-controlled mode in a first rotational speed range and accordingto a field-weakening operation in a second rotational speed range.